Verapamil and Collagen-Induced Platelet Reactions- Evidence for a Role for Intracellular Calcium in Platelet Activation

Oral verapamil inhibits platelet thrombus formation in humans

BACKGROUND

Calcium antagonists such as verapamil are potent coronary and systemic vasodilators that are used in the treatment of coronary disease. They have also been shown to inhibit platelet aggregation in vitro, but whether they have beneficial antithrombotic effects in humans is unclear, and whether they can potentiate the antithrombotic effects of aspirin is unknown.

METHODS AND RESULTS

Platelet thrombus formation and whole blood platelet aggregation were measured in 18 stable coronary patients on three separate occasions: at baseline when receiving no active medications, after 7 days of receiving oral verapamil SR (240 mg/d), and after 7 days of receiving a combination of oral verapamil SR and aspirin (325 mg/d). Thrombus formation on porcine aortic media that were placed into cylindrical flow chambers and exposed to flowing antecubital venous blood for 3 minutes was assessed morphometrically at a shear rate of 2546 s-1, which is typical of arterial flow at sites of stenoses. Thrombus formation under basal conditions was 7.0 +/- 1.6 microns 2, and this was decreased to 3.1 +/- 0.5 microns 2 (P < .05) after 7 days of treatment with oral verapamil SR and to 2.6 +/- 0.5 microns 2 (P < .05) after 7 days of treatment with oral verapamil and aspirin. Whole blood platelet aggregation levels in response to 0.050 and 0.075 U of thrombin at baseline were 10.8 +/- 1.0 and 11.9 +/- 1.0 omega; aggregation was inhibited after 7 days of treatment with verapamil to 6.5 +/- 1.1 and 7.8 +/- 0.9 omega (P < .05 versus baseline) and after 7 days of treatment with verapamil and aspirin to 6.1 +/- 1.1 and 7.2 +/- 1.0 omega (P < .05), respectively.

CONCLUSIONS

The present study demonstrates that part of the benefit of verapamil in ischemic heart disease may occur by inhibition of platelet aggregation and thrombus formation. This beneficial antithrombotic effect may be important in preventing acute coronary ischemic events resulting from thrombus formation at sites of plaque rupture.

Calcium-channel blocking agents verapamil and diltiazem are inhibitors of vasopressin-induced human platelet activation

1. This study investigated the influences of calcium-channel blocking agents verapamil and diltiazem on platelet responses induced by arginine vasopressin (AVP) and lysine vasopressin (LVP). 2. The substances inhibited platelet aggregation induced by both low and high AVP concentrations, LVP and adrenaline plus AVP. IC50 values of each drug are lower than those determined for ADP- and collagen-elicited aggregation. Verapamil and diltiazem also decreased AVP-induced thromboxane B2 synthesis. 3. Other series of experiments showed that the addition of ethyleneglycol-bis-(beta-amino-ethyl ether) N, N'-tetra-acetic acid to platelet-rich plasma samples also prevented the platelet response to vasopressin polypeptides. 4. Our data provide evidence that the effects of verapamil and diltiazem on vasopressin-induced platelet responses may be directly related to inhibition of extracellular calcium entry.

Studies on inhibition of human platelet response by diltiazem

1. Aim of the present investigation was to investigate the effects of calcium blocking agent diltiazem on human platelet response to aggregating agents. 2. Results showed that diltiazem inhibits platelet aggregation induced by ADP, arginine vasopressin, adrenaline, collagen, Na arachidonate, thrombin and phorbol ester PMA in a dose-dependent way. 3. Diltiazem decreased also beta-thromboglobulin release and Thromboxane B2 production from stimulated platelets. 4. Intraplatelet cyclic AMP levels were not modified by the substance. 5. Data provide evidence that the modulation of human platelet function by diltiazem could be also related to inhibition of protein kinase C.

Absence of (-) [3H]desmethoxyverapamil binding sites on human platelets and lack of evidence for voltage-dependent calcium channels

The two major pathways for Ca2+ entry into cells are potential-sensitive channels and receptor-operated channels. The main object of this investigation was to identify which mechanism regulates Ca2+ entry into human platelets. Platelet stimulation with thrombin, adenosine diphosphate, platelet activating factor and arachidonic acid resulted in a concentration-dependent 2.5-3-fold increase in cytoplasmic free calcium concentration over the basal levels (140 +/- 32 nM or 104 +/- 21 respectively) as measured with the fluorescent dyes Quin-2 and Fura-2. Adrenaline and collagen had no effect in promoting intracellular Ca2+ increase as measured with Quin-2 and little effect when measured with Fura-2. Incubation of Quin-2-loaded platelets with the calcium antagonists verapamil and diltiazem, which are known to inhibit Ca2+ entry from voltage-gated channels in many types of cells, over the concentration range 10(-8) - 10(-4) M did not alter significantly either the resting or the cytoplasmic free Ca2+ after stimulation of platelets by several agonists. Moreover, the calcium antagonists exhibited little or no effect on aggregation and 5-hydroxytryptamine secretion induced by platelet activating factor, adenosine diphosphate, collagen or arachidonic acid in whole blood, platelet-rich plasma or washed platelets when employed at concentration ranges as above. Similar results were obtained in washed thrombin-stimulated platelets. High doses of verapamil (but not diltiazem) inhibited platelet aggregation and secretion in response to adrenaline. Direct radioligand binding studies with (-)[3H]desmethoxyverapamil showed that platelet membranes have no receptors for this drug, suggesting that Ca2+ entry occurs in human platelets via a pathway different from potential-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclooxygenase--independent pathway of phospholipase activation in carrageenan--induced platelet aggregation

Carrageenan induced aggregation and ATP release of washed rabbit platelets. The aggregation was resistant to indomethacin or creatine phosphate/creatine phosphokinase system but sensitive to mepacrine or p-bromophenacyl bromide. Chlorpromazine, verapamil or tetracaine inhibited carrageenan-induced aggregation, ATP release reaction and membrane-bound calcium redistribution in washed platelets. Agents elevating cyclic nucleotides selectively inhibited the release reaction without affecting the aggregation. The mechanism of carrageenan-mediated platelet responses may be due to both activation of phospholipases A2 and C. Platelet activating factor, protein kinase C and inositol triphosphate or phosphatidic acid could be mediated by carrageenan to accomplish the cyclooxygenase-independent events.

Collagen increases cytoplasmic free calcium in human platelets

The role of changes in cytoplasmic free calcium in response to collagen was studied in human platelets loaded with the fluorescent calcium indicator, quin2. In the presence of 1mM external calcium, collagen caused a biphasic increase in cytoplasmic free calcium. In the absence of external calcium, there was a much smaller increase in cytoplasmic free calcium. These findings suggest that collagen increases cytoplasmic free calcium, partly by discharge of internal calcium, but mainly by stimulating calcium influx. Inhibition of cyclooxygenase by aspirin markedly reduced the second phase of the calcium response. Removal of ADP with apyrase resulted in complete inhibition of the second phase of the calcium response. The combination of apyrase and aspirin completely inhibited aggregation and the shape change caused by collagen. The calcium-entry blocking agent, verapamil, also inhibited the second phase of the calcium response to collagen. The increase in cytoplasmic free calcium is fast enough to be involved in the platelet response to collagen and these findings suggest that ADP and metabolites of arachidonic acid mediate the second phase of the calcium response to collagen.

Intracellular calcium fluxes in human platelets

Fluorescence changes and secretory responses have been measured on addition of various excitatory agonists to platelets loaded with the cytosolic Ca2+ probe, Quin 2 or with chlortetracycline as a probe for membrane-associated Ca2+. When extracellular [Ca2+] is decreased to less than 0.1 microM by addition of EGTA a linear correlation is observed between the extent of increase in cytosolic [Ca2+] and the extent of mobilisation of membrane-associated Ca2+ on stimulation by maximal doses of five excitatory agonists. A similar linear correlation between the increase in cytosolic [Ca2+] and the extent of ATP secretion is observed over the thrombin dose/response curve. Similar EC50 values are observed for ATP secretion, the increase in cytosolic [Ca2+] and the decrease in chlortetracycline fluorescence induced by thrombin. However, the decrease in chlortetracycline fluorescence shows a sigmoidal relationship with the increase in cytosolic [Ca2+] and a hyperbolic relationship with ATP secretion over this dose/response curve. Addition of prostaglandin D2 prior to thrombin causes parallel inhibition of the increase in cytosolic [Ca2+] and the decrease in chlortetracycline fluorescence induced by this agonist. However, addition of prostaglandin D2 after thrombin reverses the increase in cytosolic [Ca2+] induced by this agonist but fails to cause a similar reversal of the decrease in chlortetracycline fluorescence. The data provide further evidence supporting the proposal that chlortatracycline can be used as a probe to monitor mobilisation of membrane-associated Ca2+ but suggest that, in platelets stimulated in the effective absence of extracellular Ca2+, both Ca2+ mobilisation and Ca2+ removal can under some conditions involve sites which are not monitored by this probe.

Verapamil, an antagonist at 5-hydroxytryptamine receptors of human blood platelets

In human blood platelets verapamil and D600 (2-methoxyverapamil) in therapeutic concentrations inhibited the shape change reaction induced by 5-hydroxytryptamine (5HT) but not that induced by ADP. The N-methylated derivatives (D575 and D890) had much less effect. The inhibitory action of verapamil was independent of external Ca2+. Nitrendipine and diltiazem (20 microM) had no effect on the 5HT- and the ADP-induced shape change reactions. Since both these shape change reactions are mediated by a rise in cytoplasmic free Ca2+, it is concluded that the inhibition of the 5HT effect by verapamil and D600 was not due to their interference with calcium channels but rather to an antagonistic action on 5HT2-receptors. This view is supported by the finding that verapamil but not D575 competed with [3H]ketanserin and [3H]spiroperidol for their specific binding sites on membranes of rat cerebral cortex.

Desaggregation of PAF-acether-aggregated platelets by verapamil and TMB-8 with reversal of phosphorylation of 40K and 20K proteins

Verapamil at a concentration of 10(-4) M inhibited aggregation and release of [3H]5HT induced by platelet activating factor (PAF-acether, PAF) in rabbit platelet-rich plasma and washed labelled platelets. When added to platelets previously aggregated by PAF-acether verapamil caused them to desaggregate at doses as low as 2 X 10(-6) M. The desaggregated platelets were refractory to further additions of similar doses of PAF-acether but could further be aggregated by A23187. Simultaneous to full aggregation PAF-acether caused phosphorylation of 40K and 20K proteins in particular. Addition of verapamil at the concentration of 2 X 10(-6) M to platelets already aggregated by PAF-acether resulted in dephosphorylation of 40K protein and reduction of phosphorylation of 20K protein to the level of control parallel to desaggregation. TMB-8 (10(-3) M) also caused desaggregation and reversal of phosphorylation of 40K and 20K proteins. When A23187 was added to verapamil desaggregated platelets, 40K and 20K proteins were rephosphorylated. The extracellular calcium antagonists EGTA or La3+, when added to PAF-acether aggregated platelets, did not abolish the phosphorylation of 40K and 20K proteins. The experiments suggest that inhibition of intracellular calcium-dependent reactions is involved in the desaggregatory action of verapamil.